This invention relates to the restoration or enhancement of a protective coating on an article, and, more particularly, to the treatment of a local, discrete portion of the coating.
Certain articles, such as components operating in the hotter gas path environment of gas turbine engines are subjected to significant temperature extremes an oxidizing atmosphere and contaminants such as sulfur, sodium, calcium and chlorine which are present in the combustion gases. As a result of service operation in and exposure to such an environment, surfaces of components such as turbine blades and vanes are subject to oxidation/corrosion degradation. To protect the component substrate from excessive environmental attack, surfaces of such components normally are treated with environmentally resistant coatings widely reported in the gas turbine engine art. These environmental coatings generally are classified as diffusion or overlay coatings, distinguished by the processing methods or degree of substrate consumption during deposition.
Reported diffused aluminide coatings, relying on interdiffusion of the applied Al with the Ni from the substrate of a Ni base superalloy to create an intermetallic surface layer, have been applied by a variety of methods including pack cementation, above the pack, vapor phase, chemical vapor deposition and slurry type coating. The thickness and aluminum content of the end product coating can be controlled by varying such coating parameters and materials as the coating source materials, coating time, coating temperature and aluminum activity. For example, such control is reported in a variety of U.S. Patents including 3,544,348--Boone et al. (patented Dec. 1, 1970), and 5,658,614--Basta et al. (patented Aug. 19, 1997). The oxidation and corrosion resistant performance of diffused aluminide coatings has been shown to be enhanced by incorporating Pt, Rh and/or Pd. To incorporate these elements, thin layers of such elements generally are deposited by electroplating or physical vapor deposition means, prior to the aluminide coating cycle.
One type of overlay coating, applied for oxidation and corrosion protection of Ni base superalloy articles, includes Ni and Al along with one or more other elements such as Cr, Ta, Hf, Y and others. These coatings have been applied by deposition techniques including plasma spray, sputtering, electron beam, physical vapor deposition, among others. These processes sometimes are followed by diffusion aluminiding processes that enhance the environmental protection of the system. During heat treatments and/or service operation of the article such as in a gas turbine engine, such overlay coatings can diffuse into the substrate, consuming a portion of a load bearing wall such as the wall of a turbine blade airfoil.
Although oxidation/corrosion degradation or interdiffusion loss of original coating composition that occurs during engine service operation varies in intensity across surfaces of a turbine airfoil with some surfaces experiencing very little attack or composition loss, a current repair practice includes the complete removal from all coated surfaces of the protective diffused coatings or overlay coatings prior to repair. Such complete removal results in loss of wall thickness, from removal of the interdiffused region, which reduces component load bearing capability. Additionally, complete coating removal creates problems with maintenance of designed cooling air flow patterns and flow rates for air cooled components at points at which cooling holes, communicating with internal cooling passages, intersect the component outer surface from which the coating has been removed and must be replaced for component reuse.